MRI reveals why long-mission astronauts suffer dangerous vision problems

RSNA—Astronauts on extended missions like those involving stays in the International Space Station know to expect visual impairment that could be serious and permanent if not treated quickly upon returning to earth. Researchers now know the cause of the unfortunately predictable—and potentially preventable—problem, according to study findings presented Monday, Nov. 28, at RSNA in Chicago.

“People initially didn’t know what to make of it, and by 2010 there was growing concern as it became apparent that some of the astronauts had severe structural changes that were not fully reversible upon return to earth,” said study lead author Noam Alperin, PhD, a professor of radiology and biomedical engineering at the University of Miami in Florida, in remarks released prior to the presentation.

Alperin described how the research team used brain MRI to investigate cerebrospinal fluid (CSF) as the primary culprit, shifting suspicion away from vascular fluid flowing toward the upper body due to the forces of microgravity.

Under normal earthbound conditions, the CSF system adjusts for changes in hydrostatic pressures, such as when a seated person stands up.

“On earth, the CSF system is built to accommodate these pressure changes, but in space the system is confused by the lack of the posture-related pressure changes,” Alperin told session attendees.

To test the CSF hypothesis, he and his team scanned seven Space Station astronauts’ brains before and shortly after their missions.

They compared the results with those from nine astronauts who flew short flights on space shuttles, then used quantitative imaging algorithms in search of any correlation between changes in CSF volumes and the structures of the visual system.

Sure enough, compared to short-flight astronauts, long-mission astronauts had significantly increased post-flight flattening of their eyeballs and increased optic nerve protrusion.

In addition, the Space Station astronauts had significantly greater post-flight increases in orbital CSF volume. That’s the CSF around the optic nerves within the bony cavity of the skull that holds the eye. These astronauts also had greater post-flight increases in ventricular CSF volume in the cavities of the brain where CSF is produced.

The large post-spaceflight ocular changes observed in Space Station crew members were associated with greater increases in intraorbital and intracranial CSF volume, Alperin explained.

“The research provides, for the first time, quantitative evidence obtained from short- and long-duration astronauts pointing to the primary and direct role of the CSF in the globe deformations seen in astronauts with visual impairment syndrome,” he said.

The researchers found no significant post-flight changes of gray matter volume or white matter volume in either group of astronauts.

Identifying the origin of the space-induced ocular changes is necessary, Alperin said, for the development of countermeasures to protect the crew from the ill effects of long-duration exposure to microgravity.

“If the ocular structural deformations are not identified early, astronauts could suffer irreversible damage,” he noted. “As the eye globe becomes more flattened, the astronauts become hyperopic, or far-sighted.”

Alperin said NASA is studying a number of possible measures to simulate the conditions that lead to space-related vision problems, testing several methods for heading off the problems before they start.

RSNA 2016—aka the 102nd Scientific Assembly & Annual Meeting of the Radiological Society of North America—runs through Friday in Chicago.